Conventionally, among methods of determining a lens position corresponding to a maximum high-frequency component of a luminance signal obtained from an image sensing element such as a CCD or the like as an in-focus position used in digital still cameras, video cameras, and the like, a hill-climbing method that acquires a high-frequency component of a luminance signal (focus evaluation value) obtained from the image sensing element while moving a focus lens, and determining a position corresponding to a maximum focus evaluation value as an in-focus position by moving the focus lens in a direction to increase the focus evaluation value, and a full-area scan method that stores focus evaluation values while driving the focus lens over the entire distance measurement range, and determining a focus lens position corresponding to a maximum value of the stored values as an in-focus position, are known.
In these methods, the central portion of the image sensing frame is set as a distance measurement area, as shown in FIG. 15, and a focus lens position corresponding to a maximum focus evaluation value for an object within this range is determined as an in-focus position. The relationship between the focus lens position and focus evaluation value obtained in this way forms a mountain-like pattern, as shown in FIG. 16.
Japanese Patent No. 2,708,904 discloses an example of the method of storing focus evaluation values while moving the focus lens over the entire distance measurement range. In this example, the focus lens is moved in coarse steps from the infinity end to the closest end to acquire focus evaluation values at respective step positions. Then, the focus lens is moved in fine steps in the vicinity of the focus lens position corresponding to a maximum one of the acquired focus evaluation values to acquire focus evaluation values at respective step positions.
Japanese Patent Laid-Open No. 10-257377 discloses an image sensing apparatus which comprises first focus detection means for detecting an in-focus point while stepping a focus lens by a first drive amount, and second focus detection means for detecting an in-focus point while stepping the focus lens by a second drive amount smaller than the first drive amount, and locates the focus lens at an in-focus point detected by the first focus detection means in a normal image sensing mode or detected upon operating the second focus detection means within a predetermined range that includes an in-focus point detected by the first focus detection means in a high-image quality mode.
However, the method disclosed in Japanese Patent No. 2,708,904 is not suitable for a case wherein a focusing operation is to be quickly completed, since it scans twice per focusing operation.
Also, with the method disclosed in Japanese Patent Laid-Open No. 10-257377, since the focus detection operation is changed by switching the image sensing mode, the user must switch the image sensing mode when he or she wants to quickly complete a focusing operation, or when he or she wants to improve the focusing precision, resulting in troublesome operations.